This application claims the priority of 198 44 573.3-13, filed Sep. 29, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an engine braking process or operation for a supercharged internal-combustion engine, and more particularly, to an engine braking operation in which the engine has an exhaust gas turbocharger with a turbine with a variable turbine geometry which can be adjusted between a ram position with the smallest possible turbine cross-section and an opening position with the largest possible turbine cross-section.
DE 196 37 999 A1 discloses an internal-combustion engine which has an exhaust gas turbocharger having a turbine geometry which is variably adjustable by adjustable guide baffles. The guide baffles comprise guide blades which can be adjusted by an actuator to change the effective turbine cross-section of the turbine. As a result, depending on the operating condition of the internal-combustion engine, exhaust back pressures of different intensities can be implemented in the section between the cylinders and the turbine. Thereby, the power of the exhaust gas turbocharger can be adjusted according to the system requirements.
In order to achieve an engine braking effect in the braking operation of the internal-combustion engine, the guide baffles are changed into a ram position in which the turbine cross-section is reduced, whereby a high exhaust back pressure is built up. The exhaust gas flows at a high flow rate through the ducts between the guide blades and acts at a high impulse upon the turbine wheel. The turbine power is transmitted to the compressor. Thereupon, the combustion air fed to the engine is subjected to an increased charge pressure by the compressor.
As a result, the cylinder is acted upon on the input side by an increased pressure. On the output side, an increased exhaust back pressure exists between the cylinder outlet and the exhaust gas turbocharger. This exhaust back pressure counteracts the blowing-off of the air compressed in the cylinder into the exhaust gas pipe system. In the engine braking operation, during the compression stroke and push-out stroke, the piston must carry out compression work against the high excess pressure in the exhaust gas pipe system, whereby a strong braking effect is achieved.